2.5 Enzymes

Enzymes

Globular proteins that work as catalysts by speeding up chemical reactions without being altered themselves

Substrates

substances that bind to enzymes and are then converted into products

Active site

a special region on the surface of the enzyme to which substances bind to

Enzyme-substrate complex

Temporary complex formed when substrates bind to enzymes

Enzyme-substrate specificity

The shapes and chemical properties of an enzyme and its substrate are complementary. Therefore, only one substrate can bind to an active site, which means that the active site of an enzyme can only catalyze one biochemical reaction

Define and describe enzyme activity

The catalysis of a biochemical reaction by an enzyme

1. Substrate binds to the active site of the enzyme (some have two substrates that bind to different parts of the active site)

2. While the substrates are bound to the active site, they change into different chemical substances, becoming the reaction products

3. Products separate from the active site, leaving it vacant for substrates to bind again

Types of enzyme-catalyzed reactions

Catabolic or anabolic

Catabolic enzyme-catalyzed reaction

A single substrate is drawn into the active site and broken apart into two or more distinct molecules

Anabolic enzyme-catalyzed reaction

Two or more substrates are held in the active site, forming bonds between them and releasing a single product

Collision

The coming together of a substrate molecule and an active site

Purpose of collision

Collisions are necessary for the substrate to bind to the active site

How does collision occur?

In most reactions, the substrates are dissolves in water around the enzyme. In liquids, particles are in continual random motion. Collisions occur because of the random movement of both substrate and enzyme

Random movement of enzymes vs substrates

Both are able to move, though most substrates are smaller than the enzyme so their movement is faster

Successful collisions

Those in which the substrate and active site are correctly aligned to allow binding to take place

Factors affecting enzyme activity

Temperature, pH and substrate concentration

How does temperature affect enzyme activity?

• When heated, the particles in the liquid are given more kinetic energy. Both enzyme and substrate move around faster. Chance of substrate molecule colliding with the active site of the enzyme increases. Therefore, enzyme activity increases

• When heated, bonds in the enzyme vibrate more and the chance of the bonds breaking increases. Bond breaking causes the enzyme to denature, including its active site, so it is no longer able to catalyze reactions. As more enzymes in a solution become denatured, enzyme activity falls and eventually stops

Draw a graph of temperature vs rate of reaction

As temperature increases, so does the rate of reaction. However, as the temperature increases past the optimum point, the rate of reaction falls

How does pH affect enzyme activity?

• Each enzyme has an optimum pH, which is the pH at which enzyme activity is highest

• When [H+] is higher or lower than the optimum, the structure of the enzyme is altered, including the active site. At a very high or low pH, most enzymes are denatured, so it no longer catalyzes the reaction

Draw a graph of pH vs enzyme activity

As pH increases, so does enzyme activity. However, as pH increases or decreases from the optimum, enzyme activity falls

How does substrate concentration affect enzyme activity?

• If substrate concentration increases, substrate-active site collision will take place more frequently, and the rate at which the enzyme catalyzes the reaction increases

• After the binding of a substrate to an active site, the active site is occupied and unavailable to other substrates until the product has been formed and released

  • As substrate concentration rises, more active sites are occupied at any moment. Consequently, more substrate-active site collisions are blocked

  • Hence, the increases in rate of catalysis get smaller as concentration rises

Draw a graph of substrate concentration vs enzyme activity

As substrate concentration increases, so does enzyme activity. However, the curve rises less and less steeply, but never reaches a maximum

Catalase

One of the most widespread enzymes that catalyzes the conversion of hydrogen peroxide (a toxic byproduct of metabolism) into water and oxygen

Immobilized enzymes

Enzymes attached to another material or into aggregations, so that enzyme movement is restricted

Methods of enzyme immobilization

• Attaching enzymes to a glass surface

• Trapping them in an alginate gel

• Bonding them together to form enzyme aggregates of up to 0.1mm diameter

Advantages of immobilized enzymes

• Enzymes can easily be separated from the products of the reaction, stopping the reaction at the ideal time and preventing contamination of products

• Enzymes can be recycled after being retrieved from the reaction mixture, saving costs as many enzymes are very expensive

• Immobilization increases the stability of enzymes to changes in temperature or pH, reducing the rate at which they are degraded and have to be replaced

• Substrates can be exposed to higher enzyme concentrations than dissolved enzymes, speeding up reactions

Lactose

sugar naturally present in milk

Lactase

Enzyme that converts lactose into glucose and galactose

How is lactase obtained?

Obtained from a type of yest that grows naturally in milk. Biotech companies culture the yeast, extract the lactase and purify for sale

Advantages of lactase in food processing

• Reduces lactase in milk, allowing lactose-intolerant people to consume it

• Galactose and glucose are sweeter than lactose, so less sugar needs to be added to sweet foods containing milk

• Lactose tends to crystallize during the production of ice cream, giving it a gritty texture. Glucose and galactose are more soluble than lactose, so they remain dissolved, giving a smoother texture

• Bacteria ferment glucose and galactose more quickly than lactose, so the production of yogurt and cottage cheese is faster